1. Field of the Invention
The present invention relates to an optical head for performing at least recording or reproduction of information on or from, e.g., an optical disk or an optical memory by radiating a light beam thereon and, more particularly, to an apparatus for detecting a focusing error of an objective lens with respect to the recording surface of an optical memory.
2. Description of the Related Art
An optical head is used for an optical disk apparatus for recording/reproducing information using, e.g., an optical disk. In this optical head, in order to focus and radiate light from a light source onto a predetermined recording surface of an optical disk, it is important to cause the focal position of an objective lens to coincide with the recording surface. For this purpose, the focusing error of the objective lens relative to the recording surface is detected, and the objective lens is moved in the direction of the optical axis on the basis of the focusing error (i.e., focusing servo control).
A large number of methods of detecting a focusing error in an optical head have already been known. The method of the present invention is most similar to a method of double-knife-edge. FIG. 40 shows the arrangement of an optical system in a focusing error detecting apparatus employing the method of double-knife-edge.
Referring to FIG. 40, light reflected by an optical disk (not shown) passes through an objective lens (not shown) in the opposite direction to light incident on the optical disk, and is subsequently separated from the incident light by a beam splitter 101. Thereafter, the separated light is split into two light beams by wedge prisms 102 and 103 (cf. or two-split prisms). The two light beams are then focused by a converging lens 104 onto the detection surfaces of two-split photodetectors 105 and 106 arranged at different positions.
FIGS. 41A to 41C are projected views showing changes in spot shape of light beams on detection surfaces 105a, 105b, 106a, and 106b of the two-split photodetectors 105 and 106. FIG. 42 is a graph showing curves indicating changes in focusing error signal and in reproduced information signal with respect to focusing errors. If output signals from the detection surfaces 105a, 105b, 106a, and 106b of the two-split photodetectors 105 and 106 are respectively represented by A, B, C, and D, a focusing error signal obtained by a processing circuit can be represented by (A+D)-(B+C); and a reproduced information signal, (A+B+C+D). As is understood from the curves in FIG. 42 , in the method of double-knife-edge, changes in level of a focusing error signal near the in-focus point are large, thus ensuring high detection sensitivity of focusing errors.
In this conventional focusing error detecting apparatus, however, as shown in FIG. 41B, since very small light beam spots are formed on the detection surfaces of the photodetectors 105 and 106 in the just focusing condition, the widths of non-detecting regions on the detection surface splitting lines of the photodetectors 105 and 106 are significantly large in relation to the sizes of the beam spots. The amounts of light incident on the photodetectors 105 and 106 and a focusing error detection range in the just focusing condition are substantially determined by the widths of non-detecting regions. Therefore, the degree of freedom in setting such amounts of light and detection range is greatly decreased.
As described above, in the conventional focusing error detecting apparatus based on the method of double-knife-edge, since the amounts of light incident on the photodetectors and the focusing error detection range in the just focusing condition are greatly influenced by the widths of the non-detecting regions of the photodetectors, the degree of freedom in setting them is low. Therefore, the degree of freedom in setting in design and applications is undesirably decreased.